The field of the invention is stereolithography, and more particularly improved stereolithography methods and apparatus for manufacturing parts or objects more rapidly, reliably, accurately, and economically.
It is common practice in the production of plastic parts or objects to first design the part and then produce a prototype of the part. This requires considerable time, effort, and expense. For example, tooling or molds may be required, even to produce just the prototype. The design is then reviewed and often times the laborious and expensive process is again and again repeated until the design has been optimized. After design optimization, the next step is production. Most production plastic parts are injection molded. Since the design time and tooling costs are very high, plastic parts are often only practical in high volume production. While other processes are available for the production of plastic parts, including direct machine work, vacuum-forming and direct forming, such methods are typically only cost effective for short-run production, and the parts produced are usually inferior in quality to molded parts.
Techniques have been developed in the past for making three-dimensional objects within a fluid medium. These techniques involve selectively curing the medium, e.g., a resin, with a beam of radiation. U.S. Pat. Nos. 4,041,476; 4,078,229; 4,238,840 and 4,288,861 describe some of these techniques. All of these techniques or systems rely on the buildup of synergistic energization, or curing energy, at selected points deep within the volume of the fluid medium, to the exclusion of all other points in the fluid volume. These systems however, encounter a number of problems with resolution and exposure control. The loss of radiation intensity and image forming resolution of the focused spots of the beam, as they are directed deeper into the fluid medium, create complex beam control situations. Absorption, diffusion, dispersion and diffraction all contribute to the difficulties of working deep within the fluid medium on an economical and reliable basis.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330, which patent is incorporated herein by reference as if set forth herein in full, have come into use. Stereolithography is a method for automatically building simple or complex parts (e.g., plastic parts) by successively "printing" cross-sections or layers of a solidified fluid-like building material on top of each other, with all of the layers joined together to form a whole part. The building material may be, for example, a photopolymer which is solidifiable upon exposure to UV radiation or the like. Powder material, which forms a solidified mass when sintered by conducted or radiated heat from a heated element or source of IR radiation or the like, and powders which are solidifiable by the addition of a reactive chemical such as a binder, may also be utilized. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes.
One type of useful fluid medium, photocurable polymers (photopolymers) change from a liquid to solid when exposed to light. Their photospeed under ultraviolet light (UV) is fast enough to make them practical building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. In one embodiment an ultraviolet laser generates a small intense spot of UV. This spot is moved across the liquid surface with a galvanometer mirror X-Y scanner. The scanner is driven by computer generated vectors. After each successive surface is exposed by the laser, an elevator lowers the object further into the vat and allows another layer of fresh liquid to cover the surface of the object for formation of the next layer. Precise complex patterns can be rapidly produced with this technique.
The laser, scanner, photopolymer vat and elevator, along with a controlling computer and possibly a separate computer for creating appropriate cross-section data from initially supplied three-dimensional object data, combine together to form a stereolithography apparatus, referred to as an "SLA." An SLA is programmed to automatically make a part by drawing its various cross-sections, one layer at a time, and building the part up layer-by-layer.
Stereolithography does not use tooling, molds, dies, etc. Since it depends on using a computer to generate cross-sectional layers or patterns, an SLA can be readily linked (i.e., a data link) to computer aided design as a computer aided manufacturing (CAD/CAM) apparatus.
Many photopolymers have a "minimum solidifiable thickness," i.e., a minimum thickness below which they cannot be sufficiently cured to form unsupported regions of transformed, cohesive material. For example, with presently preferred fluid photopolymers, if an attempt is made to try to form a feature of an object having a thickness less than the minimum solidifiable depth (MSD) or thickness, that feature will either simply fail to sufficiently solidify to become part of the object, or it will slump (i.e., fail to hold its shape) when the object or individual layer is moved relative to the vat of fluid photopolymer. The minimum solidifiable thickness of a building medium (e.g. photopolymer) is not only a characteristic of the building medium or material itself but it also depends on the synergistic stimulation source chosen (e.g. the solidifying radiation such as ultraviolet light) and the environmental conditions surrounding the material. For example, oxygen absorbed in a photopolymer can act as a reaction inhibitor. Therefore, as used herein, "MSD" refers to the minimum solidification depth obtainable with a given material/solidification environment combination. The minimum solidification depth can also be considered the depth resulting from the minimum exposure that is preferred for curing down-facing features of an object what ever the basis for this preferred minimum. It may be based on a desire to form a minimum solidified thickness of material from a single layer, which minimum thickness is selected for its ability to withstand curl distortion or to supply sufficient structural integrity. These definitions can apply to any fluid-like material whether liquid, powder, paste, emulsion, or the liquid. Furthermore, these definitions can also apply to building material that is applied in sheet form and then transformed.
Many liquid building materials also have a Minimum Recoating Depth, MRD, or thickness; i.e. a minimum coating thickness that can reliably be formed over previously solidified material. This minimum recoating depth, may derive from a dewetting phenomena that occurs between the liquid material and the previously solidified material. Alternatively, the MRD may simply be based on apparatus or process limitations regarding the timely formations of coatings; in other words, the minimum thickness may be set by a maximum acceptable recoating time or accuracy limitation. This alternative definition can be applied to both liquid and powder materials.
Stereolithography makes objects layer by layer. Since the MSD is the minimum solidification depth for forming unsupported regions of layers (i.e., down-facing features of the object), these regions must be given a cure depth of at least the MSD regardless of the thickness between individual layers or cross-sections from which the object is being formed. Therefore, due to the layer-by-layer formation process, even if the layers being used are thinner than the MSD, the accuracy of the stereolithographically reproduced object is limited by the MSD of the material being used.
Moreover, because of the layer-by-layer formation process of stereolithography, the MRD sets the minimum coating thicknesses that can be effectively utilized by standard stereolithographic techniques. This minimum coating thickness directly sets the vertical accuracy obtainable when using standard stereolithographic techniques.
In the remainder of the specification, the MSD and MRD may be expressed in either (1) particular units of length or (2) without units or followed by an "LT". "LT" is an acronym for "layer thickness" and in particular the thickness for layers having the desired resolution. If the MRD is expressed without units, or followed by an LT, it should be understood that it is expressed in terms of the number of layers, each possessing the desired resolution, that yield a thickness equal to the MRD.
Accordingly, it is an object of the invention to overcome the MSD limitation by providing a method and apparatus of practicing high resolution stereolithography when using a building material that is inherently incapable of making unsupported thicknesses of solidified material as thin as the desired level of accuracy when solidified by the chosen synergistic stimulation.
Additionally, it is accordingly an object of the invention to overcome the MRD limitation by providing a method and apparatus of practicing high resolution stereolithography when using a fluid-like building material that is inherently incapable of reliably forming coating over previously solidified material as thin as the level of accuracy desired when forming the object.
Another object of this invention is to provide a method and apparatus for enhancing object production by horizontally distinguishing regions of an object, to be formed, based on their distance from a given point, points, surface, or surfaces so as to form those regions with different building parameters.
Another object of this invention is to provide a method and apparatus for enhancing object production by vertically distinguishing regions of an object, to be formed, based on their distance from a given point, points, surface, or surfaces so as to form those regions with different building parameters.
A further object of the invention is to provide a method and apparatus for automatically performing "Z-error correction", i.e., correcting for the condition in which the relative distance between an up-facing and down-facing feature of an object produced through stereolithography is greater than a desired amount due to the MSD or MRD being greater than the desired LT of a layer situated between the up and down facing features.
Another object of the subject invention is to provide improved methods of representing an object which facilitate Z-error correction, the display of the object on a graphical display device which is not equipped to display a fine level of detail regarding the object, (e.g. insufficient processing power to display object and its movement in a timely manner) and various manipulations to the object representation such as scaling-up or scaling-down operations.
An additional object of the invention is to provide a method and apparatus for reducing rounding errors (e.g. errors associated with rounding of polygon vertices of NURB control points to slicing planes).
Further objects of the invention include utilization of the above objects alone or in combination with any two or more of the above objects into combination methods and combination apparatus to provide further enhancements to stereolithography. Other objects, useable alone or in combination, will be apparent to one of skill in the art from the teachings found herein.